Insulation resistance on a lighting circuit

[rustynails]

Today I got 0.11 Mohm L-N>E at 250v then the same at 500v on a 6amp lighting circuit wired in the 70s which is the only lighting circuit for the house. Customer did not want me to remove the lamps. I forgot to ask or see a bell transformer or aerial booster. No faults known to the customer. MCB not tripping. No RCD. I broke the circuit down and tested L-N at 250v dead short 0.0 Then L-E N-E at 250v and 0.11 for both... I keep telling myself it must be something connected... Or is it a fault. Indicated further investigation on the MWC. Spending tonight racking over what it could be. Also how do you guys explain this to a customer without waffling the entire bs7671 to them whilst using a new piece of cable to explain cable deterioration and the benefit of having an RCD with out them thinking that your having them on.

 

[HandySparks]

Any outside lights on the circuit?

 

[telectrix]

the L-N reading will be through the load/s. as tis reading is too low for an IR test to read, you'll get the same readings L-E and N-E. to determine which it is, you need to remove the load/s/lamps etc

 

[Silly Sausage]

Also how do you guys explain this to a customer without waffling the entire bs7671 to them whilst using a new piece of cable to explain cable deterioration and the benefit of having an RCD with out them thinking that your having them on.

I find the best thing to do in these situations, is to suggest sitting down with a brew, then slipping an E in their tea, after you've just pointed out something behind them.
Twenty minutes later...they'll agree to anything.

 

[Octopus]

Aerial amp in the loft? RCD FCU on the circuit? Shaver point(s)?

 

[rustynails]

Two, I had one disconnected which was the reason to be there and the other halogen flood had the lamp in high up.

more than likely to be a load then...

 

[VoltzElectrical]

If the wiring was installed in the 70's, what were you doing there to justify the MWC as you haven't actually stated what you did, just what you found?

 

[rustynails]

Replacing an outside PIR bulkhead with a b and q blooma twin spot pir.

 

[marc_v33]

General maintenance then?

 

[rustynails]

General maintenance then?

If that's what you like to call it. Some people call it work and I feel lucky to have that work to do which allows me to eat and sit on the couch to watch liverpool get another dicking...

 

[7029 dave]

L N, dead short get them lamps out, an look for anything else in circuit.

 

[HandySparks]

Two, I had one disconnected which was the reason to be there and the other halogen flood had the lamp in high up.

more than likely to be a load then...

Assuming that the above is in answer to "Any outside lights?", I would suggest that the first place to look for the 0.11M L+N to E is an existing outside light with water ingress.

 

[Lucien Nunes]

tested L-N at 250v dead short 0.0

0.0 what? Ohms? Megohms? Why would you call it a dead short?

"0.0" means 0 +/- 0.05. If that's the number in the window of the MFT that could be +/- a few counts, let's be generous to the MFT and say one count. So if you mean megohms, all we know is that the reading is less than 150 kilohms, which will pass a spectacular 1.5mA at 230V and consume 1/3 of a watt. So far from being a dead short (which would have tripped the MCB) it could be a load so tiny that it wouldn't light a torch bulb and half a dozen of them running to earth wouldn't trip an RCD.

By dead short I would infer two pieces of wire touching - 0.0 ohms is more like it!

Now 0.11 megs to earth is a fault. Whichever way you look at it, that is a fault. See above re. wet stuff...